1. Field of the Invention
The present invention relates to a shadow mask for a color cathode ray tube and a manufacturing method thereof. More particularly, the present invention relates to a shadow mask having a shadow mask thin plate formed of a material having an assembling structure that allows electron beam passing slots thereof to be uniform during etching treatment, and a method for forming the shadow mask by adjusting the assembling structure of the material forming a thin plate thereof so as to enable uniform etching and to decrease deformation during fabrication, thereby preventing generation of a doming phenomenon.
2. Discussion of Related Art
With the development of transmitting systems having an increased number of scanning lines, cathode ray tubes must have high quality image screens and must be able to represent color with high definition.
FIG. 1 is a sectional view illustrating the construction of a representative cathode ray tube. Panel 1 covers fluorescent film 3 on the inner surface thereof, and panel 2 covers a conductive graphite material on the inner surface thereof. Panel 1 and panel 2 are coupled to each other using a fusing glass (not shown). Electron gun 6, which generates electron beam 5, is mounted on neck portion 4 of panel 2. Shadow mask 7, which is supported by frame 8, is positioned on the inner side of panel 1. Deflection yoke 9, which deflects the electron beam 5 in the left and right direction, is mounted on the peripheral surfaces of panel 2, respectively. Inner shield 10, which is provided to prevent the advancing path of electron beam 5 emitted from electron gun 6 from being deviated due to earth magnetic field or a leakage magnetic field, is secured by the frame 8.
If the video signal is input to electron gun 6, thermion is emitted from a cathode of the electron gun 6 toward panel 1. The emitted thermion is accelerated and concentrated by the application of a voltage to each electrode of electron gun 6.
An advancing path of the electron beams emitted from electron gun 6 is adjusted according to a magnetic field generated by deflection yoke 9 mounted on neck portion 4 of panel 2. The adjusted electron beams are scanned across the front surface of panel 1, while being protected from being deformed by inner shield 10. The deflected electron beams pass through slots of shadow mask 7 coupled with the inner side frame of panel 2. After passing through shadow mask 7, the deflected electron beams are finally incident on, and used to heat, the screen (not shown) at predetermined locations. Then, the incident electron beam collides with fluorescent film 3 of the inner surface of panel 1, causing light emissions which represent the image signal.
Generally, shadow mask 7 is used with a rimmed steel or an aluminum killed steel under the specification defined by JISG 3141.
With the recent development of a extremely fine pitch resolution, however, a thermal expansion coefficient of the shadow mask is increased to 11.5 .times.10.sup.-6 deg.sup.-1. As a result, heat generated due to the collision of shadow mask 7 with the electron beams emitted from electron gun 6 causes the shadow mask 7 to experience thermal-expansion. When the shadow mask expands or contracts, the electron beams passing through the shadow mask may be incident upon a portion of the fluorescent screen other than the predetermined portion of the fluorescent screen. As a result, a color broadening phenomenon, that is, a doming phenomenon, appears undesirably on the screen. This problem is recognized as a serious problem in displays, such as televisions and the like, which pursue a high fine pitch and luminance screen.
Therefore, to control the generation of doming phenomenon, shadow masks are sometimes formed of a material having a low thermal expansion characteristic, such as Fe--Ni Invar alloy (Ni contents of 36% and Fe contents of 64%) having a thermal expansion coefficient of 1.5.times.10.sup.-6 deg.sup.-1 (see JPA 61-78033 and JP Patent Publication No.62-174353 ).
The shadow mask made of Invar alloy is etched to form electron beam passing slots on the mask plate, which is a thin plate. After several hundreds of thousands to millions of slots are formed, the shadow mask is subjected to a heat treatment and to a forming process in which it is shaped into a curved surface.
In more detail, the thin plate of 0.1 to 0.2 mm in thickness is sequentially subjected to de-greasing, cleaning, photoresist covering, exposure, developing, etching, photoresist film removal, and cutting processes. Furthermore, after completion of the etching process, it is also subjected to annealing, press forming, blackening, welding assembly and packing processes.
Since the Fe-36% Ni alloy contains a great amount of Ni, the cost of the material is relatively high and the etching and pressing quality is deteriorated, as compared with an aluminum killed steel such as a low carbon steel.
Since the Invar alloy exhibits relatively poor etching characteristics, it is difficult to achieve a successful fine pitch operation. Therefore, if a fine pitch is needed, the plate should be designed to be considerably thin. If the plate is designed to be thin, it will lack rigidity after the press forming process is performed, which leads to a weak resistance to the impact against the color cathode ray tube.
In addition, the thin plate is comprised of a specified structure having an effective surface on which the electron beam passing slots are formed, a non-effective surface around the effective surface, and a skirt portion on which the electron beams are not passed. The process used to form this structure is complicated, leading to potential errors.
The thermal expansion characteristic of an Invar alloy is only 1/7 to 1/10 the thermal expansion characteristic of a pure steel. Thus, the low thermal conductivity characteristics combined with the high specifics resistance of Invar alloys lead to reduction or improvement of only about 1/3 of the doming generation in the color cathode ray tube where the Invar alloy is used.
It is therefore considered necessary to improve the etching and mechanical forming qualities of the Invar shadow mask.
JP Patent Publication No. 62-174353 suggests a method for improving the etching and press forming characteristics of the Invar alloy. Specifically, boron(B) is admixed to the Invar alloy to improve {100} assembling structure, and simultaneously chromium(Cr) is admixed to the Invar alloy to reduce breakdown strength. When the thin plate is etched and blackened after the admixture of boron, however, there is a disadvantage that the, blackened film exhibits a low adhesion characteristics.
From these low adhesion characteristics follow various problems. For instance, the blackened film is used to suppress temperature increment in the shadow mask and to improve a thermal radiation characteristic thereof.
Therefore, the thermal radiation characteristic of the Fe-36% Ni alloy--a material of the shadow mask, is seriously reduced if the adhesion of the blackening film is deteriorated. Thus, the doming phenomenon due to increment of temperature and the thermal expansion of the shadow mask is likely to occur if boron is admixed, as suggested by Japanese Patent Application No. 09-56107.
Also, when the blackening film has low adhesion characteristics, uneven surface layers of the blackened Fe-36% Ni Invar alloy result.
Furthermore, other problems are experienced by films using Invar alloys, notwithstanding the addition of boron(B). Specifically, the Fe-36% Ni Invar alloy has a high breakdown strength, causing a spring-back to occur after forming. Also, high precision formation techniques are not generally executed on a predetermined curved surface. Thus, qualities of the products formed are unpredictable.
During the forming process, a warm forming method is generally employed. However, because the relationship between the shadow mask and the fluorescent material screen is not matched, a color broadening phenomenon occurs on the screen.
In addition, to mold the shadow mask made of a Fe--Ni alloy into a structure that has precise lines, the processes used to form the shadow mask, including the annealing process and the process for adjusting the thickness of the thin plate material, are typically managed.
Even if the processes for forming the shadow mask are managed, it is difficult to achieve a mold of the shadow mask of Fe--Ni alloy that is sufficiently precise.